The widely accepted answer is Vshojo, but to write off Phase Connect (or any other non-Vshojo agency) so quickly is foolish. Aside from the CCV comparisons others were making, I wouldn't be surprised if Selen and Pomu (and perhaps Mika) are being courted with sweetheart deals from other agencies. This is about money in the end, so if they end up going with a smaller company which will result in a CCV cut but they get an amazing split, they could come out ahead in the end.
That said, I don't think Pomu will end up at Phase, if she's still interested in doing the idol thing. Phase just doesn't have the experience and facilities to do that sort of thing, even though they look like they're trying to move in that direction with ASLUR and the new music channel. (So the same thing Aze just said above but in my defense he added that while I was still drafting my own reply.) So maybe Hololive would be a really good fit for her. I think Selen could be just fine at PC. And given that the environment around Vshojo has changed (for the better) in the past year maybe she'd still be able to collab with her friends in Vshojo without the fans at either corpo getting too pissy about it.
Despite her now somewhat regular collabs with PC I don't think Sayu has any plans to go corporate in the near future seeing as how she just recently fundraised and then purchased a new model, new assets, a nice new lore video, etc. It'd be silly for her to go through all that trouble and expense if she's been trying to get back into a corpo this whole time, and if she did it soon it might piss off a good number of her donors (and rightly so).
Inspired by what Porean shared in my stream chat today, let's have a look at the subscriber counts of NijiEN's graduated and terminated members.
Mysta: 1.17M
Pomu: 623k
Nina: 600k
Yugo: 415k
Zaion: 108k
Total: 2.916M